Gates, Microsoft's co-founder, joined the ranks of those on the microblogging site, where posts are limited to 140 characters, on Tuesday with this tweet: "'Hello World.' Hard at work on my foundation letter — publishing on 1/25."

Gates remains part-time chairman of the software giant after stepping down as chief executive officer in mid-2008 to devote his time to the Bill and Melinda Gates Foundation, a philanthropic effort devoted to improving health and education worldwide. (Msnbc.com is a joint venture of Microsoft and NBC Universal.)

While there are many celebrities on Twitter, there are also many celebrity imposters. Gates' Twitter account — @BillGates — is noted as being "verified." That means he's the real deal, according to Twitter.

Among the first to welcome Gates to Twitter was actor Ashton Kutcher, almost the de-facto ambassador for Twitter, which has become more prominent in the past year. Last spring, Kutcher was the first on Twitter to have more than 1 million "followers," or people subscribing to his tweets, or posts.

"Thanks to you and all the other people who have welcomed me," Gates tweeted to Kutcher. "I've got a lot to learn about Twitter but look forward to sharing more."

In Day One on Twitter, as word spread about Gates being on the site, his number of followers quickly went to more than 60,000.

Gates also has rejoined Facebook, the social networking site. He quit Facebook about six months ago, after being inundated with thousands of "friend" requests.